The goal of this project is to determine the role of lung cyt P-450 in mediating lung injury in the newborn premature infant at risk for BPD. Aim 1 is to determine if the cyt P-450 inhibitor cimetidine will reduce early lung injury, as reflected by pulmonary gas exchange, in human premature infants at risk for BPD. Aim 2 is to determine the degree of lipid peroxidation in study patients by measuring F2-isoprostane levels in tracheobronchial aspirates and in urine. Aim 3 is to identify P-450 mediated arachidonic acid (AA) metabolites in tracheobronchial aspirates obtained from patients enrolled in the study and to correlate the levels of specific metabolites with the assigned treatment and gas exchange index. Aim 4 is to identify inflammatory markers in tracheobronchial aspirates and urine obtained from study patients in order to determine whether the anticipated reduction in O2 free radical generation and P-450 AA metabolite production in patients receiving cimetidine is also accompanied by a reduced inflammatory response.